style(rust): document declaration ordering

src/skills/style/rust/rules/declarations.md

@@ -0,0 +1,14 @@
+# Declaration rules
+
+## Order declarations by use
+
+### Rule
+
+Order related Rust declarations from the type that uses other local types to those local dependencies.
+For a type with multiple local field dependencies, place dependencies in field order.
+Apply the rule depth-first before moving to the next dependency.
+
+### Rationale
+
+Use-first order lets readers start with the public or owning shape.
+They can then read supporting types in the order the owner references them.

src/xatu/file_store.rs

@@ -1,14 +1,5 @@
 use std::path::PathBuf;
 
-#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
-pub struct FileId(usize);
-
-#[derive(Debug)]
-pub struct SourceFile {
-    pub path: PathBuf,
-    pub source: String,
-}
-
 #[derive(Debug, Default)]
 pub struct FileStore {
     files: Vec<SourceFile>,
@@ -31,3 +22,12 @@ impl From<Vec<SourceFile>> for FileStore {
         Self { files }
     }
 }
+
+#[derive(Debug)]
+pub struct SourceFile {
+    pub path: PathBuf,
+    pub source: String,
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub struct FileId(usize);

src/xatu/rules.rs

@@ -6,6 +6,12 @@ use crate::rules::empty_inherit::EmptyInherit;
 pub mod deprecated_to_path;
 pub mod empty_inherit;
 
+#[derive(Debug)]
+pub struct Diagnostic {
+    pub file_id: FileId,
+    pub finding: Finding,
+}
+
 #[derive(Debug)]
 pub struct Finding {
     pub rule_id: &'static str,
@@ -13,12 +19,6 @@ pub struct Finding {
     pub message: &'static str,
 }
 
-#[derive(Debug)]
-pub struct Diagnostic {
-    pub file_id: FileId,
-    pub finding: Finding,
-}
-
 impl std::fmt::Display for Diagnostic {
     fn fmt(&self, formatter: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         write!(